The present invention relates to a lithium secondary battery which can be easily fabricated, has small internal resistance, and is superior in operational safety.
Attention has been paid to realization of practical use of a lithium secondary battery as a secondary battery which is small, has a large energy density, and can be charged and discharged, and which can be used as a power source for an electric equipment such as a portable communication equipment and a notebook-sized personal computer, the electric equipment being rapidly miniaturized in recent years. Moreover, concerns for resource saving and energy saving are raised on the background of international protection of the earth environment, and that is one of the reasons why the lithium secondary battery is expected as a motor driving battery for an electric vehicle which is under consideration for the positive introduction to the market in the automobile industry, or also as an effective means for using electric power by storing the electric power in the night. Thus, it is eagerly desired to put a large capacity lithium secondary battery, which is suitable for these uses, into early practical use.
In the lithium secondary battery, a lithium transition metal compound oxide or the like is used as a positive active material, while a carbon material such as hard carbon or graphite is used as a negative active material. At charging, lithium ions in the positive active material are transferred to and captured by the negative active material through an electrolyte obtained by dissolving a lithium ion electrolyte in an organic solvent. At discharging, a reverse battery reaction occurs.
Like this, the lithium secondary battery is a secondary battery which can be charged and discharged, and has properties that its voltage is higher than a conventional manganese battery and its energy density is high. Thus, the lithium secondary battery is provided with a safety mechanism for preventing accidents such as a burst of the battery caused from the rise of a battery temperature by abnormalities at the time of charging or discharging, for example, over-discharging due to a short circuit of an output terminal, rapid charging or over-charging due to malfunction of a charging device, application of a reverse connection potential by erroneous use of a user, and the like.
In a small battery as an example, as shown in FIG. 13, one end of a positive lead line 11 is connected to a positive electrode (not shown), and the other end thereof is connected to an internal terminal 13 having a communicating hole 12. The internal terminal 13 is electrically connected to a pressure switch plate 15 having a burst groove 14 through contact B. The pressure switch plate 15 is connected to an output terminal 17 through a PTC element 16. The internal terminal 13 and the pressure switch plate 15 are isolated from each other through an insulator 18 so that when the contact B is separated by the increase of internal pressure of the battery, the electrical conduction is lost. Reference numeral 19 denotes a battery case.
In the battery having such a safety mechanism, in the case where the temperature of the battery itself is raised due to the occurrence of an abnormality in the state of use of the battery, when the PTC element 16 reaches a predetermined temperature, the resistance thereof is abruptly increased so that current hardly comes to flow. As a result, the battery reaction is restrained and the rise of temperature is suppressed.
Here, as this kind of PTC element 16, a mixture of conductive particles and polymer is generally used. The mixture has such properties that at room temperature, the mixture has low resistance since the conductive particles form a current path, while at a temperature higher than a certain temperature, the mixture comes to have high resistance close to an insulator since the molecular structure of the polymer is changed so that the current path formed of the conducive particles is cut to pieces, and when the temperature lowers again, the polymer returns to the original structure so that the current path of the conductive particles is again formed and the resistance is returned to a low resistance value.
If the internal pressure of the battery is increased although a current is restricted by the PTC element 16, and the internal pressure exceeds the welding strength of the contact B between the pressure switch plate 15 and the internal terminal 13, the contact B separates so that the connection between an internal electrode body and the output terminal 17 is completely cut off and the battery reaction comes not to occur. However, if the internal pressure is increased in spite of this, the burst groove 14 bursts so that the internal pressure of the battery is released to the atmospheric pressure, that is, such a structure is adopted that the pressure release mechanism operates.
In addition to the above described various safety mechanisms, such a safety mechanism is provided that by using a porous macromolecular film, such as polyethylene with a low softening point, as the separator for separating the positive electrode from the negative electrode in the internal electrode body, when the battery temperature is raised, the separator film is softened and micro pores formed in the film are collapsed so that the movement of lithium ions is blocked and the battery reaction is suppressed.
Accordingly, also in a large capacity lithium secondary battery, it is conceivable that such various safety mechanisms as are installed in a small lithium secondary battery as described above must be indispensably installed.
However, since the resistivity of the above-mentioned PTC element made up of the conductive particles and polymer is about 1 xcexa9xc2x7cm at room temperature, the internal resistance of the battery becomes large to cause output loss, and the PTC element may cause the discharge characteristics to lower and the lifetime of the battery to shorten. Especially, in the case where such a PTC element is installed in a large capacity battery, the concentration of current in the inside of the PTC element is apt to occur due to the increase of an area of the PTC element, which causes heat generation, so that the installation to a large capacity battery is difficult. In addition, such a PTC element is generally expensive, and a large one in size is not manufactured, so that a current control element which is more inexpensive, is capable of dealing with a large battery, and has low resistance, is earnestly desired.
According to the Battery Association of Japan, as a mechanical test (erroneous use test) of a lithium secondary battery safety estimation guideline, it is regulated that even if an abnormal discharge current abruptly flows by an internal short circuit of electrodes which is caused by driving a nail (metal rod) in a surface (lamination surface) so that the nail pierces vertically electrode plates of a lithium secondary battery, which is fully charged in the charging capacity, the electrode plates being overlapped with each other on the surface, the battery does not burst, does not fire, and the safety can be secured (such a test will be hereinafter referred to as a nail piercing test).
As a result of the foregoing nail piecing test carried out by the present inventors for a small lithium secondary battery (battery capacity 1.5 Ah or less), it was confirmed that safety was able to be sufficiently secured for the battery in which a pressure releasing mechanism was provided at an electrode side of only one end.
However, in the case where the nail piecing test was carried out for the below-mentioned lithium secondary battery which has a wound cylinder-shape and a large capacity, employs an inner electrode body, and was made an airtight structure and in which a pressure releasing mechanism was provided at one end and the pressure release mechanism was not provided at the other end, it was confirmed that the pressure release mechanism was normally operated at the end of the battery where the pressure releasing mechanism was provided, and a burst groove burst so that the internal pressure of the battery was released to the atmospheric pressure and a small amount of vapor of an electrolyte flowed out. On the contrary, it was confirmed that the end where the pressure release mechanism was not provided, burst so that not only partial pieces of the battery case and the vapor of the electrolyte, but also partial materials of the positive electrode or negative electrode were scattered.
The present inventors presumed that the result of the nail piecing test for the large capacity lithium secondary battery was caused since an overcurrent at the short circuit was larger than that of the small battery beyond comparison, vaporization or decomposition of the electrolyte due to the temperature rise of the battery was rapidly progressed so that the internal pressure of the battery was raised. On the other hand, the small lithium secondary battery in which the pressure release mechanism was provided only at one portion, met the nail piecing test. From these results, it is conceivable that the structure and installation state of a pressure releasing mechanism which operates without fail is related to the battery capacitance.
However, the relation between the battery capacitance and the opening area at the operation of the pressure release mechanism has not been clarified. That is, if the opening area at the operation of the pressure release mechanism is small, clogging occurs on the way to the pressure release of the internal pressure, so that the pressure release is not sufficiently carried out and there is a fear that such an accident as burst or firing of the battery would occur. On the other hand, if the opening area is large, although the fear of clogging can be removed, there is a fear that the constituents of the internal electrode body would jump out, or firing or combustion would occur when the internal electrode body jumps out in the state of the internal short circuit. However, conditions for preventing the occurrence have not been clear.
Thus, there are often cases where the battery case itself becomes large since w an unnecessarily large pressure release mechanism for battery capacity is disposed, or such restriction in configuration is imposed that although a battery of thin and long cylindrical shape is desired to be formed, a flat plate structure must be adopted while a battery capacity remains, since a large pressure release mechanism must be disposed.
Then the present inventors considered the structure, operational condition, and installation position of a pressure release mechanism for a large capacity lithium secondary battery with superior safety and low resistance, which is able to release a large pressure generated in the inside of the battery due to an electrode short circuit and the like to the atmospheric pressure, and as a result, the present invention has been achieved.
According to an aspect of the present invention, there is provided a lithium secondary battery, comprising:
a battery case;
an internal electrode body contained in the battery case and including a positive electrode, a negative electrode, and a separator made of porous polymer, the positive electrode and the negative electrode being wound through the separator so that the positive electrode and the negative electrode are not brought into direct contact with each other;
an organic electrolyte contained in the battery case; and
a pressure release mechanism disposed at each of both end portions of the battery case in a winding direction of the positive electrode and the negative electrode.
Such a structure of the lithium secondary battery is preferably adopted for the case where a cylinder-shaped battery case is used. At least one pressure release mechanism is suitably disposed in each of the end portions of the battery case.
Further, according to another aspect of the present invention, there is provided a lithium secondary battery, comprising:
a battery case;
an internal electrode body contained in the battery case and including a positive electrode, a negative electrode, and a separator made of porous polymer, the positive electrode and the negative electrode being laminated through the separator so that the positive electrode and the negative electrode are not brought into direct contact with each other;
an organic electrolyte contained in the battery case; and
at least one pressure release mechanism disposed on a side surface, perpendicular to flat surfaces of said positive electrode and said negative electrode, of the battery case.
The pressure release mechanism is preferably disposed on each of at least one pair of facing side surfaces of the battery case.
In the foregoing lithium secondary battery of the present invention, when the battery capacity is C (Ah) and the total area of opening portions where the pressure release mechanisms operate is S (cm2), it is preferable that the lithium secondary battery is so designed that the relation of 0.05xe2x89xa6S/Cxe2x89xa62 is established. In addition, it is preferable that the operation pressure of the respective pressure release mechanisms is 2 to 10 kg/cm2, and the difference between the operational pressures of the respective pressure release mechanisms is 8 kg/cm2 or less.
According to still another aspect of the present invention, there is provided a lithium secondary battery, comprising:
a battery case;
an internal electrode body contained in the battery case and including a positive electrode, a negative electrode, and a separator made of porous polymer, the positive electrode and the negative electrode being wound through the separator so that the positive electrode and the negative electrode are not brought into direct contact with each other;
an organic electrolyte contained in the battery case; and
at least one pressure release mechanism disposed in one end portion of the battery case in a winding direction of said internal electrode body,
wherein when the total area of an opening portion where the pressure release mechanism operates is S (cm2) and capacity of the lithium battery is C (Ah), the relation of 0.5xe2x89xa6S/Cxe2x89xa62 is established.
According to yet another aspect of the present invention, there is provided a lithium secondary battery, comprising:
a battery case;
an internal electrode body contained in the battery case and including a positive electrode, a negative electrode, and a separator made of porous polymer, the positive electrode and the negative electrode being laminated through the separator so that the positive electrode and the negative electrode are not brought into direct contact with each other;
an organic electrolyte contained in the battery case; and
at least one pressure release mechanism disposed on a side surface perpendicular to flat surfaces of said positive electrode and said negative electrode or on each of at least two of not facing side surfaces, of the battery case,
wherein when the total area of an opening portion where the pressure release mechanism operates is S (cm2) and capacity of the lithium battery is C (Ah), the relation of 0.5xe2x89xa6S/Cxe2x89xa62 is established.
In the foregoing all lithium secondary batteries of the present invention, it is preferable to install the pressure release mechanism having such a structure that a metal foil is burst, or a groove portion is formed in a metal plate and the groove portion is burst, so that the internal pressure of the battery is released to the outside pressure. Moreover, it is preferable to use aluminum for the metal foil or metal plate used in this way when it is installed at the positive side, and copper or nickel when it is installed at the negative side. Moreover, it is preferable that an opening area of such a pressure release mechanism is 0.1 cm2 or more.
Such structural condition of the lithium secondary battery of the present invention is preferably adopted for a battery having a battery capacitance of 5 Ah or more, and can be preferably used for an electric vehicle or a hybrid electric vehicle.
As described above, according to the lithium secondary battery of the present invention, not to mention an internal short circuit, even in the case where a battery temperature is raised by overcharging due to an external short circuit or the like so that the internal pressure of the battery is increased, since the pressure release mechanism is disposed in a suitable shape for the battery capacitance and at a suitable place, the entire battery does not burst or explode, and superior safety is obtained. Moreover, even in the case where the pressure release mechanism functions also as a current path, since the pressure release mechanism is made of metal material members, the internal resistance of the battery is small and the battery is superior in charging and discharging characteristics.